1. Field of the Invention
The present invention relates to a plasma source which supplies a plasma to an ion beam and neutralizes the ion beam by electrons of the plasma to prevent the target from being charged, in an ion beam irradiation apparatus (e.g., an ion implanting apparatus) for radiating an ion beam toward a target, more particularly, relates to a means for preventing high energy electrons from being contained in the discharged plasma, and the ion implanting apparatus using the plasma source.
2. Description of the Related Art
In carrying out a process, e.g., ion implanting process, to implant into a targets for example, a semiconductor wafer, by radiating an ion beam to the target, a problem arises in which the target is positively charged (charged up) by the positive charge of the ion beam.
In order to solve the charge-up problem, JP-A-5-234562 and JP-A-5-47338 disclose that a plasma source is located near a target. Plasma is supplied to an ion beam. The ion beam is neutralized by electrons in the plasma, to thereby prevent the charging of the target. Incidentally, the plasma source used for such purpose is also called an ion beam neutralizer.
The plasma source will be described with reference to a drawing. FIG. 6 shows an apparatus in which a conventional plasma source is attached to an ion beam irradiation apparatus.
In the apparatus, an ion beam 34 is irradiated on a target 36 that is held by a holder 38 within a vacuum container 32, whereby a process of ion implanting, ion beam etching or the like is applied to the target 36. The target 36 is a semiconductor wafer, another substrate, or the like.
As shown in FIG. 6, a hole 33 is formed in a side wall of the vacuum container 32, which is located near the target 36. A plasma source 2 is attached to a portion on the vacuum container 32, which is located near the hole 33, while an insulating member 30 is interposed therebetween.
The plasma source 2 of the ECT (electron cyclotron resonance) type is composed of a plasma chamber 4 of metal for generating a plasma 20, a gas introducing pipe (gas introducing means) 10 for introducing a gas 12 for plasma generation, such as argon or xenon, into the plasma chamber 4, an antenna (microwave introducing means) 14 of metal for introducing a microwave 15 at 2.45 GHz into the plasma chamber 4, and a magnetic coil 18 for generating along a plasma emission direction 22 a magnetic field B having an intensity high enough to cause an electron cyclotron resonance within the plasma chamber 4 (the intensity: approximately 87.5 mT when the frequency of the microwave 15 is 2.45 GHz) Reference numeral 16 is a connector.
To make it easy to extract the plasma 20 into the vacuum container 32, provision is preferably made of means for applying a DC voltage (extraction voltage) from a DC extraction power supply 28 to between the plasma chamber 4 and the vacuum container 32 in a state that a positive polarity of the extraction voltage is set at the vacuum container 32.
The front side of the plasma chamber 4 consists of a front board 6 with a plasma emission aperture 8 in this instance. A plasma 20 that is efficiently generated by microwave discharging and electron cyclotron resonating operations within the plasma chamber 4 flows out through the plasma emission aperture 8 into the vacuum container 32, and is supplied to the ion beam 34 (this is called a plasma bridge.). Through the plasma bridging operation, the ion beam 34 is neutralized by the electrons in the plasma 20, to thereby suppress formation of positive charges at the target 36, which results from the ion beam irradiation.
The plasma source 2 uses the microwave 15, not the filament, for the generation of the exposure unit 20. Therefore, there is no fear that the target 36 is contaminated with the materials of the filament that are sputtered out through the plasma emission aperture 8.
Further, the inner wall of the plasma chamber 4 and the antenna 14 are covered with insulating covers 24 and 26, respectively. With use of the covering, there is no fear that the metal plasma chamber 4, the front board 6 and the antenna 14 are sputtered by the plasma 20 to thereby contaminate the target 36.
In the thus constructed plasma source 2, when energy of electrons contained in the plasma 20 is high, the high energy electrons reach the target 36 and possibly charges negatively (charges up) the target up to a voltage corresponding to the electron energy. Some technical measure should be taken for the negative charging of the target 36.
Recently, the transistors (FETS) formed in the surface of the target wafer are remarkably reduced in size (e.g., one side of each of them is about 0.18 .mu.m), and its gate oxide film is extremely thinned (e.g., about 5 nm). For this reason, it is necessary to set the charge-up voltage at an extremely low voltage (e.g., about 5V or lower). Otherwise, the charge-up voltage causes the breakdown of the transistors, possibly resulting in reduction of a production yield in fabricating the transistors and deterioration of product reliability.
In the plasma source 2 mentioned above, within the plasma chamber 4, the magnetic field B for the electron cyclotron resonance is generated along the plasma emission direction 22 in which the plasma flows out through the plasma emission aperture 8. Accordingly, the electrons in the plasma 20 within the plasma chamber 4 are accelerated by the electron cyclotron resonance along the magnetic field B. The high speed (i.e., high energy) electrons are extracted through the plasma emission aperture 8 since the plasma emission aperture 8 is located along the acceleration direction of the electrons. Those electrons are supplied to the ion beam 34, and reaches the target 36. As a result, a problem of increase of the charge-up voltage of the target 36 arises.
The energy of the electrons in the exposure unit 20 supplied from the plasma source 2 is distributed over a range from several eV to 100 eV, as will be described in detail later referring to FIG. 4. Therefore, the charge-up voltage of the target 36 could be increased up to a value near 100V at maximum.